1. Field of the Invention
The present invention relates to a method for driving an AC plasma display panel (hereinafter referred to as PDP) having a surface discharge structure.
PDPs have been becoming widespread as large-screen TV display devices since PDPs capable of color display were put to use. Now still higher definition PDPs are one of the market's demands. For realizing higher definition, it is necessary to speed up addressing.
2. Description of Related Art
Three-electrode AC surface-discharge PDPs are commercialized as color display devices. In a PDP of this type, a pair of main electrodes (a first electrode and a second electrode) for sustaining light emission is disposed per line (row) of a matrix for display and an address electrode (a third electrode) is disposed per column of the matrix. Ribs for preventing discharge interference between cells are formed in stripes. In the surface-discharge structure, fluorescent layers for color display are formed on a substrate opposed to a substrate on which the pairs of main electrodes are disposed. Thereby deterioration of the fluorescent layers by ion impact at discharges can be reduced and thus the life of the PDP can be extended. PDPs of reflection types which have the fluorescent layers on their rear substrates are superior in luminous efficiency to those of "transmission type" which have the fluorescent layers on their front substrates.
A memory function is utilized for display. The memory function is attributed to charge accumulated on a dielectric layer covering the main electrodes. More particularly, addressing is performed by line-by-line scanning for producing a charged state according to the content of display, and a sustain voltage Vs of alternating polarity is applied on the main electrode pair of each line for sustaining illumination. The sustain voltage satisfies the following formula (1): EQU Vf-Vwall&lt;Vs&lt;Vf Formula (1)
wherein Vf is a firing voltage and Vwall is a wall voltage.
When the sustain voltage is applied, an effective voltage (also referred to as a cell voltage) exceeds the firing voltage only in cells where wall charge is present, so that a surface discharge is generated along the face of the substrate in the cells. By applying the sustain voltage Vs in a short cycle, it is possible to obtain an illumination state which appears continuous.
The luminance of display depends on the number of discharges per unit time. Accordingly, halftones are reproduced by setting the number of discharges in one field for every cell in accordance with levels of gradation to be produced. Color display is one sort of gradation display, and a displayed color is determined by combination of luminances of the three primary colors. In the present specification, the "field" means a unit image, and a number of unit images are displayed in time sequence for reproducing an image. That is, the field is a field of a frame displayed by interlaced scanning in the case of television and is a frame itself in the case of non-interlaced scanning (which is regarded as a one-to-one interlaced scanning) typified by computer output.
In order to produce levels of gradation by the PDP, the field is time-sequentially divided into a plurality of sub-fields. The luminance (i.e., the number of discharges) in each sub-field has a weight. The total number of discharges in the field is determined by combining illuminations and non-illuminations on a sub-field basis. If the application cycle (driving frequency) of the sustain voltage Vs is constant, the sustain voltage Vs is applied for different time periods for different luminance weights. Basically, the sub-fields are assigned so-called "binary weights" represented by 2.sup.q (q=0, 1, 2, 3, . . . ). For example, if the number K of sub-fields in one fields is 8, 256 (2.sup.8) levels of gradation from "0" to "255" can be produced. The binary weights are free of redundancy and suitable for multi-gradation display. In some cases, however, different sub-fields are purposely assigned the same weight for preventing pseudo-contour with moving pictures or the like.
A method in which plural lines are simultaneously driven is known as a method for driving PDPs to realize the gradation display.
FIG. 10 is a time chart of the multi-line simultaneous driving method, explaining the outline of the timing of selecting lines. Here, for simplicity of explanation, an example of display of 16 levels of gradation with four bits is shown and a screen has a line number n of 480. The abscissa of the time chart represents time and the ordinate thereof represents the position of pixels in a direction of rows on the screen. Oblique solid and dotted lines in the time chart represent scanned positions, i.e., selected lines, at points in time. The screen is a set of lines to be scanned and is sometimes equal to part of a set of cells arranged in matrix. For example, in the case of a dual scanning method in which the addressing is executed separately to two sections into which the cells are divided in a direction of columns, each of the sections is a screen. In the case where the addressing is executed separately to even lines and to odd lines, the set of even lines and that of odd lines each compose a screen.
A field time Tf divided by the number n of lines, Tf/n, is a scanning time period H (line selecting time period) for scanning one line in each of the sub-fields sf1, sf2, sf3 and sf4. The sub-fields sf1 to sf4 are assigned binary weights of 1:2:4:8, respectively. Accordingly, time allotted to the sub-fields sf1 having the weight of "1" is 32(=1.times.480/(1+2+4+8))H. Times allotted to the sub-fields Sf2, sf3 and sf4 are 64H, 128H and 256H, respectively. The addressing and the sustaining of illumination are executed within these times allotted to the sub-fields sf1 to sf4.
In the example of FIG. 10, display of the sub-fields sf1 to sf4 is executed in order of the weights. The lines are selected from the first line to the last line in order of arrangement. In other words, from the view point of each line, the first line is selected for the sub-field sf2 32H after selected for the sub-field sf1. Then 64H later, the line is selected for the sub-field sf3, and then 128H later, the line is selected for the sub-field sf4. Further 256H later, the line is selected for the sub-field sf1 of the next field. The second line is selected 1H after the first line is selected, and the third line is selected 1H after the selection of the second line. Thus, the lines are selected at intervals of 1H in order of their arrangement in each of the sub-fields sf1 to sf4.
In this way, from the view point of the individual lines, the selection of a line is 1H behind the selection of the previous line in each of the sub-fields sf1 to sf4. However, from the view point of the whole screen, the selection of lines for the four sub-fields sf1 to sf4 is performed in the period of 1H. More particularly, as indicated by black dots in the figure, when a first line is selected in the sub-field sf1 of a certain field, the selection of lines for the sub-fields sf2, sf3 and sf4 of the previous field is also performed. In other words, four lines are selected at the same time on the whole screen. At this time, the selected four lines are apart from each other by the numbers of lines corresponding to the weights of luminance assigned to the sub-fields sf1 to sf4. In the example shown in FIG. 10, selected are the first line, the 257th line apart from the first line by 256 lines, the 385th line apart from the 257th line by 128 lines, and the 449th line apart from the 385th line by 64 lines. As discussed above, the selection of lines proceeds one line per 1H. Therefore, when the second line is selected, the 258th line, 386th line and 450th line are selected.
In the multi-line simultaneous driving method, the same number of lines as the number K (k is four in the example) into which the lines are divided are selected at the same time. Actually, since it is impossible to simultaneously address a plurality of lines using one address electrode, the selection of lines for the sub-fields sf1 to sf4 is performed in time-sequential order within the period of 1H.
FIG. 11 and FIG. 12 illustrate voltage waveforms explaining conventional driving sequences.
In the conventional driving sequences, a sustain pulse Ps for sustaining illumination is alternately applied to pairs of main electrodes Xi and Yi (i=1 to n) of the lines with a timing common to all the lines, and a scan pulse Py is applied to the main electrode Yi with such a timing that the scan pulse Py does not overlap the sustain pulse Ps.
When the field is divided into four, the scan time period H for scanning four lines for the sub-fields sf1 to sf4 is divided into four, and the scan pulse Ps is applied to one line within the period of 1/4H. Though not shown, an address pulse is selectively applied to the address electrode in synchronization with the scan pulse. Thus, only in a cell on the selected line which is on a column to which the address pulse is applied, an address discharge is generated to produce a wall charge. The Examples of FIGS. 11 and 12 explain a write addressing. Accordingly, the wall charge is erased by an erase pulse Pe prior to applying the scan pulse Py, and the address discharge produces the wall charge again in such an amount as required for sustaining illumination. In the cell where the wall charge has been re-produced, a discharge for sustaining illumination is generated and switch the polarity of the wall charge every time the sustain pulse Ps is applied, until the erase pulse Pe is applied next.
The conventional driving sequences have the problem that a cycle (H/k) for applying the scan pulse Py is larger than the sum of the pulse width of the scan pulse Py and double the pulse width of the sustain pulse Ps and therefore the addressing takes a long time. For this reason, the conventional driving sequences cannot be adapted to a high-definition PDP having more than 480 lines for producing full-motion display with sufficient levels of halftone. In this connection, it is possible to halt the application of the sustain pulse Ps for a while, during which the scan pulse Ps is applied, and thus to reduce the time necessary for the addressing. In this case, however, it is necessary to separately control not only the main electrode Yi to which the scan pulse is applied but also the other main electrode Xi. Accordingly, the driving circuit becomes more complicated and costs more compared with the case where the main electrodes Xi are controlled in common.